1. Field of the Invention
This invention relates to molded case circuit breakers and more particularly to a side plate, used with the circuit breaker operating mechanism, having twist tabs extending downwardly from the bottom edge for connecting the side plates to the circuit breaker base.
2. Description of the Prior Art
Molded case circuit breakers are generally old and well known in the art. Examples of such circuit breakers are disclosed in U.S. Pat. Nos. 4,489,295; 4,638,277; 4,656,444 and 4,679,018. Such circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload and relatively high level short circuit condition. An overload condition is normally about 200-300 percent of the nominal current rating of the circuit breaker. A high level short circuit condition can be 1000 percent or more of the nominal current rating of the circuit breaker.
Molded case circuit breakers include at least one pair of separable contacts which may be operated either manually by way of a handle disposed on the outside of the case or automatically in response to an overcurrent condition. In the automatic mode of operation, the contacts may be opened by an operating mechanism, controlled by an electronic trip unit, or by magnetic repulsion forces generated between the stationary and movable contacts during relatively high levels of overcurrent.
In one automatic mode of operation, the contact assemblies for all poles are tripped together by an electronic trip unit and a mechanical operating mechanism. More particularly, the electronic trip unit is provided with current sensors to sense an overcurrent condition. When an overcurrent condition is sensed, the current transformers provide a signal to the electronic circuitry within the electronic trip unit to actuate the operating mechanism to cause the main contacts to be separated.
In the other automatic mode of operation, the contact arm assemblies are disengaged from the mechanical operating mechanism and are blown open by magnetic repulsion forces. More particularly, magnetic repulsion members or shunts are used to allow the contact arm, which carries the movable main contact, to pivot. Each magnetic repulsion member is generally V-shaped defining two legs. During relatively high level overcurrent conditions, magnetic repulsion forces are generated between the legs of the magnetic repulsion member as a result of current flowing through the legs in opposite directions. At a relatively high level overcurrent condition, these magnetic repulsion forces cause the contact arm carrying the movable main contact to be blown open.
During a blow open condition, each contact arm is operated independently of the mechanical operating mechanism. For example, for a three phase circuit breaker having a high level overcurrent on the A phase; only the A phase contact arm will be blown open by its respective repulsion member. The contact arms for the B and C phases would remain closed and thus are unaffected by the operation of the A phase. The contact arms for the B and C phases are tripped by the electronic trip unit and the operating mechanism. This is done to prevent a condition known as single phasing, which can occur for circuit breakers connected to rotational loads, such as motors. In such a situation, unless all phases are tripped, the motor may act as a generator and contribute to the overcurrent condition.
The operating mechanism for the circuit is disposed adjacent the center pole. Side plates are used to pivotally support several components of the operating mechanism, such as the crossbar. The side plates must be connected to the circuit breaker base very securely in order to avoid affecting the proper operation of the operating mechanism. In known breakers, the side plates are connected to the base of the circuit breaker by various means, such as tabs with threaded ends, spun over ends or stakes received in load bearing plates carried by the circuit breaker base.
In other known circuit breakers, side plates having downwardly extending tabs with straight shank portions and enlarged head portions are provided. These tabs are received in slots disposed in spin plates, carried in the underside of the base. The twist tabs are twisted to secure the side plates to the base. In this design, it is necessary to control the length of the shank in order to avoid play in the side plates after the twist tabs are twisted to avoid affecting operation of the operating mechanism.